Sensitive electronic components and integrated circuits are susceptible to electromagnetic interference, EMI. The presence of electromagnetic signals therefore often interferes with the operation and performance of sensitive electronic components. When there exists electromagnetic interference, there also exists a source of EMI, a receptive component as a victim of EMI and a coupling path through which a source emission conducts to a receptive component. Certain electronic components emit electromagnetic signals during their operation. Such electromagnetically emitting components may affect the performance and operability of other components for example on the same circuit board. The EMI also degrades the reliability of components and circuits. Certain components can act both as EMI sources and as receptive components. Between a source and a receptive component there exists a so-called coupling path.
An EMI source emission can be a conducted voltage or current coupling via conducted paths through which current can flow, or an electric or magnetic field coupling through space or materials through electromagnetic wave propagation. So, the transmission of EMI can occur across a circuit or through the air due to electric field and/or magnetic field propagations. Also the electromagnetic waves can propagate through circuit boards or substrate materials. EMI signals may occur due to some electromagnetic radiator, electric- or magnetic fields or failures during the design, such as sharing of conductors with EMI sources.
For protecting the sensitive electronic components and to minimize the presence of interfering signals, an EMI shielding is employed. An EMI shielding is used for protecting electronic components from both interfering sources on the same assembly and components apart from the arrangement. Shielding cans may be applied to prevent interfering signals from being emitted by an emitting source, or to inhibit interfering signals from propagating to a receptive component. An EMI shielding can prevent electromagnetic waves, which propagate through space, by absorbing or reflecting waves incident on walls of a shielding can. Material of a shielding can is advantageously selected to be reflective and/or absorbtive so that most of interfering EMI signals or waves are reflected from and/or absorbed by the shielding walls. Shielding walls, then advantageously absorb a transmitted part of interfering waves, so a part is not reflected off. Only some residual noise or energy is then able to pass through shielding walls. This residual noise or energy forms the resulting EMI. So the essential characteristics are the reflectance and absorbance properties of a shielding can. These characteristics depend on shielding material and an interface of a shielding with a base substrate and circuit ground.
The interfaces inside the shielding and the grounding are essential parts of the shielding concept. Typically there has to be some openings in the shielding walls to form an input and output for certain wanted signals of the system. So the shielding arrangement is never perfect (closed), but it must include openings as input and output paths for signals. The problem is, that also the unwanted signals tend to use these paths, since signal lines of printed circuit boards typically act like antennas. Through these signal lines external noise gets inside the shielded environment and the signals of the system are affected and may drop out.
Another problem in shielding sensitive electrical components is grounding. Grounding and generally a grounding concept is one issue, which typically reduces shielding efficiency and disturbs the system. It is typical that printed circuit boards are stacked in order to make the arrangement as compact as possible. Usually printed circuit boards are contacted both mechanically and electrically to a base or a frame supporting the arrangement. Electrical circuits of the printed circuit board and a frame typically have one common ground potential. There are a few attempts in the prior art to avoid EMI effects by using separate ground potentials. If these different grounds of electrical circuits and the frame get into contact with each other, there may occur a ground loop between printed circuit boards. The ground loop affects the operation of the printed circuit boards by exposing those to annoying noise, which in turn distorts the circuit ground to a value over zero. This affects the proper operation of circuits.
In the patent publication U.S. Pat. No. 6,011,691 there are pronounced separate ground potentials of the printed circuit boards and the chassis. The ground potentials are separated so that a cover is added to the top and bottom of the electronic component assembly to provide a Faraday shield. Further the cover contacts a plated area around the periphery of the electronic component assembly and ensures an outer chassis ground potential and an inner ground potential of the heat sink and printed circuit boards do not come into contact. This solution is based on an inner core and a dielectric material covering at least part of the core, on which the printed circuit boards and chassis are arranged to be assembled.